Its done on a crank balancer. It basically is a big drill press so you can drill the counter weights (remove weight) Matching the counter weights on the crank with the weight of the rods,pistons,rings,wrist pin and rod bearings

Most LS engines are pretty balanced within 10 grams or so I have seen a few 30 grams out
The lighter the rotating assembly the quicker the revs can be and the smoother it will run and cuts down on wear...]
I BALANCE ALL CRANKS WITHIN 2 grams

Alot of aftermarket cranks mostly when the rods and pistons are light weight you have to add weight to the counter weights